The present invention relates to optical fiber transmission.
The refractive index profile of an optical fiber is generally described as a function of the appearance of the graph of the function that associates the refractive index with the radius of the fiber. Conventionally, distance r to the center of the fiber is plotted along the x-axis, and difference between refractive index and the refractive index of the cladding of the fiber is plotted up the y-axis. Thus, the terms xe2x80x9cstepxe2x80x9d, xe2x80x9ctrapeziumxe2x80x9d, or xe2x80x9ctrianglexe2x80x9d profiles are used for graphs respectively having step, trapezium or triangle shaped profiles. The curves are generally representative of the ideal or reference profile of the fiber, it being possible for the stresses induced during manufacture of the fiber to give rise to a profile that is significantly different.
In new high data rate and wavelength division multiplexed (WDM) networks, it is advantageous to control chromatic dispersion, in particular for data rates greater than or equal to 10 gigabits per second (Gbit/s), so that, for all of the wavelength values of the multiplex, chromatic compensation is obtained over the link that, cumulatively, is substantially zero, so as to limit the spreading of the pulses. A cumulative value of a few hundred ps/nm is generally acceptable for the dispersion. In the vicinity of the wavelengths used in the system, it is also advantageous to avoid zero values for the chromatic dispersion, since non-linear effects are then greater. Finally, it is also advantageous to limit the chromatic dispersion gradient over the range of the multiplex so as to avoid or to limit distortion between the channels of the multiplex.
Conventionally, step-index fibers are used as the line fibers for optical fiber transmission systems. Under reference ASMF 200, the Applicant sells a step-index monomode fiber having a chromatic dispersion zero wavelength xcex0 in the range 1300 nm to 1320 nm, and chromatic dispersion that is less than or equal to 3.5 ps/nm.km in a range from 1285 nm to 1330 nm, and that is 17 ps/nm.km at 1550 nm. The chromatic dispersion gradient at 1550 nm is about 0.06 ps/nm2.km.
Dispersion shifted fibers (DSF) have also appeared on the market. Those fibers are such that, at the transmission wavelength at which they are used, which is in general different from the 1.3 xcexcm wavelength for which the dispersion of silica is substantially zero, the chromatic dispersion of the transmitted wave is substantially zero. In those fibers, the index difference xcex94n between the core of the fiber and the optical cladding is increased relative to step-index optical fibers. That index difference makes it possible to shift the wavelength for which chromatic dispersion is zero towards the transmission wavelength; it is obtained by inserting dopants into the preform while said preform is being manufactured, e.g. by means of a Modified Chemical Vapor Deposition (MCVD) process which is known per se and not described in any more detail herein.
NZ-DSF+ is used to designate non-zero dispersion shifted fibers that have non-zero and positive chromatic dispersion for the wavelengths at which they are used. For those wavelengths, such fibers have low chromatic dispersion, typically less than 10 ps/nm.km at 1550 nm, and chromatic dispersion gradients in the range 0.04 ps/nm2.km to 0.1 ps/nm2.km.
In order to compensate for chromatic dispersion and for the chromatic dispersion gradient in SMF (single mode fiber) or NZ-DSF+ fibers used as line fibers, it is known that short lengths of dispersion-compensating fiber (DCF) can be used.
DCF fibers are described in various patents. In the vicinity of a wavelength of 1550 nm, they have negative chromatic dispersion so as to compensate for the cumulative chromatic dispersion in the line fiber, and, in addition, they can have negative chromatic dispersion gradients so as to compensate for the positive chromatic dispersion gradient of the line fiber.
Document WO-A-99 13366 proposes a dispersion-compensating fiber that is intended for use in compensation boxes for compensating the chromatic dispersion and the chromatic dispersion gradient of a fiber of the type sold by Lucent Technologies under the xe2x80x9cTrue Wavexe2x80x9d trademark. The xe2x80x9cTrue Wavexe2x80x9d fiber has chromatic dispersion in the range 1.5 ps/nm.km to 4 ps/nm.km and a chromatic dispersion gradient of 0.07 ps/nm2.km. In the range 1530 nm to 1610 nm, the proposed dispersion-compensating fibers have chromatic dispersion of less than xe2x88x926 ps/nm.km, a chromatic dispersion gradient of less than xe2x88x920.6 ps/nm2.km, and a ratio between those two values of less than 160. In order to compensate the dispersion in the line fiber, the dispersion-compensating fiber is used in a compensation box, the length of DCF used being 15 times shorter than the length of the line fiber.
French Patent Application filed on Feb. 18, 1999 under number 99 02 028, published under number FR-2 790 107, and entitled xe2x80x9cFibre de ligne pour systxc3xa8mes de transmission à fibre optique à multiplexage en longueurs d""ondexe2x80x9d [xe2x80x9cLine fiber for WDM optical fiber transmission systemsxe2x80x9d] proposes a line fiber specially suited to dense wavelength division multiplexed transmission, with inter-channel spacing of 100 GHz or less for a data rate per channel of 10 Gbit/s. For a wavelength of 1550 nm, that fiber has an effective area greater than or equal to 60 xcexcm2, chromatic dispersion lying in the range 6 ps/nm.km to 10 ps/nm.km, and a chromatic dispersion gradient of less than 0.07 ps/nm2.km.
Amongst the fibers of document WO-A 99 13366, and in particular amongst those described in the examples in that document, nothing points to the fibers offering the best compromise for compensating the chromatic dispersion and the chromatic dispersion gradient of NZ-DSF fibers, and in particular of the fibers described in document FR-2 790 107. In other words, it is not possible in document WO-A 99 13366 to determine the dispersion-compensating fibers which offer the best compromise between bend losses, effective area (in order to avoid non-linear effects), chromatic dispersion, and chromatic dispersion gradient.
The invention proposes a fiber suitable for in-line compensation of chromatic dispersion in a dispersion-shifted fiber, and more precisely in an NZ-DSF+ fiber, in particular in a fiber of the type described in document FR-2 790 107. It provides a fiber that has bend losses which are low, and that is easy to use as a line fiber in a transmission system.
More precisely the invention provides an optical fiber which, for a wavelength of 1550 nm, has chromatic dispersion that is negative and greater than xe2x88x9240 ps/nm.km, a chromatic dispersion to chromatic dispersion gradient ratio that is in the range 50 nm to 230 nm, the optical fiber being characterized in that is has an effective area that is greater than or equal to 10 xcexcm2; bend losses that are less than or equal to 0.05 dB; and an ideal cutoff wavelength that is greater than or equal to 1.1 xcexcm. The ideal cutoff wavelength is the calculated wavelength beyond which only the fundamental mode can be propagated (for more details, reference can be made to the work of L. B. Jeunhomme, entitled xe2x80x9cSingle-Mode Fiber Optics, principles and applicationsxe2x80x9d, 1990 edition, pages 39 to 44).
Choosing the ideal cutoff wavelength beyond 1.1 xcexcm enables the desired compromise to be obtained.
In a preferred embodiment, for a wavelength of 1550 nm, the fiber has chromatic dispersion that is greater than or equal to xe2x88x9220 ps/nm.km.
In another preferred embodiment, for a wavelength of 1550 nm, the fiber has chromatic dispersion that is less than or equal to xe2x88x925 ps/nm.km.
In another preferred embodiment, for a wavelength of 1550 nm, the fiber has a ratio between chromatic dispersion and chromatic dispersion gradient that is less than 200 nm, and that is preferably less than 180 nm, and that is more preferably less than 160 nm.
In another preferred embodiment, for a wavelength of 1550 nm, the fiber has a ratio between chromatic dispersion and chromatic dispersion gradient that is greater than 80 nm, and that is preferably greater than 100 nm, and that is more preferably greater than 120 nm.
In another preferred embodiment, for a wavelength of 1550 nm, the fiber has an effective area that is greater than or equal to 15 xcexcm2, and preferably greater than or equal to 20 xcexcm2.
In another preferred embodiment, for a wavelength of 1550 nm, the fiber has attenuation that is less than or equal to 0.3 dB/km.
In another preferred embodiment, for a wavelength of 1550 nm, the fiber has a mode diameter that is greater than or equal to 4 xcexcm, and preferably greater than or equal to 5 xcexcm. In another preferred embodiment, for a wavelength of 1550 nm, the fiber has a sensitivity to micro-bends that is less than or equal to 1, and preferably less than or equal to 0.5.
In another preferred embodiment, the fiber has an ideal cutoff wavelength that is greater than or equal to 1.3 xcexcm.
In another preferred embodiment, the fiber has an in-cable cutoff wavelength that is less than or equal to 1.3 xcexcm.
The cable cutoff wavelength is the cutoff wavelength measured on a fiber that is 20 m long (for more details reference can be made to the EIA/TIA-455-170 standard).
In another preferred embodiment, for wavelengths lying in the range 1300 nm to 1620 nm, the fiber has bend losses that are less than 0.05 dB, and preferably less than 5xc3x9710xe2x88x923 dB, for a winding of 100 turns with a radius of 30 mm.
In another preferred embodiment, the fiber has a rectangular index profile with a depressed zone and a ring.
In another preferred embodiment, the difference between the index at any point of the fiber and the index of the cladding is greater than or equal to xe2x88x928xc3x9710xe2x88x923.
In another preferred embodiment, the difference between the index at any point of the fiber and the index of the cladding is greater than or equal to 28xc3x9710xe2x88x923, and preferably less than or equal to 20xc3x9710xe2x88x923.
The invention also provides the use of a fiber of the invention as a dispersion-compensating fiber in a wavelength division multiplexed optical fiber transmission system. Preferably, the dispersion-compensating fiber is part of a cable and used as a line fiber.
The invention further provides a wavelength division multiplexed optical fiber transmission system including a first segment of line fiber, and a second segment of line fiber of the invention.
In a preferred embodiment, for a wavelength of 1550 nm, the line fiber of the first segment has chromatic dispersion lying in the range 5 ps/nm.km to 11 ps/nm.km.
In another preferred embodiment, for a wavelength of 1550 nm, the line fiber of the first segment has a chromatic dispersion gradient less than or equal to 0.08 ps/nm2.km.
In another preferred embodiment, the ratio of the length of the first segment to the length of the second segment is substantially the inverse of the absolute value of the ratio between the chromatic dispersions at 1550 nm of the fibers of the first segment and of the second segment.
In another preferred embodiment, the cumulative chromatic dispersion for each channel in the range 1530 nm to 1620 nm is less than 100 ps/nm, and preferably less than 50 ps/nm, on average over 100 km of transmission.